Coated ferrous metal article and method of preparing same



United States Patent COATED FERROUS METAL ARTICLE AND METHOD OFPREPARING SAME Robert A. Parson, Dyer, Ind., and John Truhlar, ClarendonHills, Ill., assignors to Pullman-Standard Car Manufacturing Company,Chicago, Ill., a corporation of Delaware No Drawing. Application April6, 1954 Serial No. 421,445

4 Claims. (Cl. 148-615) The present invention relates to coatings formetals which prevent corrosion and provide other advantages, and moreparticularly to methods of providing such coatings and to the coatingsprovided thereby.

In general, in accordance with the invention, a metal surface is treatedto form a coating of a complex mixture of phosphates of the metalthereon which not only prevents corrosion of the metal but dissipates orremoves corrosion previously present. The coating also providesa mosteffective bond for paints and the like which it may be desired to applyto the surface, and itself forms a decorative as Well as protectivefinish for the surface. In addition, the coating increases the lubricityof the metal surface and facilitates metal forming operations, such asdrawing and cold extrusion. The coating adheres very tightly to themetal, being integrally bonded to the base, is continuous, very denseand uniform, of greater Weight than it has heretofore been possible toattain and impervious to corrosive agents. To achieve the superiorcoating of this invention, the metal is treated with a preferablyaqueous solution of certain acid pyrophosphates or other substanceswhich yield the hereinabove is obtained simply, economically, and in ashort period of time. By the method of the invention, a ferrous metalsurface, for example, may readily be provided with a phosphate coatingof a weight Well in excess of one hundred milligrams per square foot ofsurface, and the weight may easily be increased to five or six hundredmilligrams, and even more, no precise upper limit having been found. Incontrast, prior iron phosphate coatings on ferrous metals have normallybeen limited to not more than fifty to seventy-five milligrams persquare foot for production applications, and in addition to being lighthave usually been non-uniformly formed and required accelerators andother materials to be employed in addition to the phosphatizingsubstance.

ice

The invention employs an acid pyrophosphate selected from the group ofsodium, potassium, lithium, ammonium, and amine pyrophosphates toprovide the corrosion-preventing, paint-bonding coating on metal. Thesepyrophosphates are preferred because of their relative cheapness,availability, solubility in water, safety, and ease of handling. Othermaterials or substances which contain or include these acidpyrophosphates, or from which such pyrophosphates may be derived, ormore accurately substances which yield the pyrophosphate negativeradical, such as the polyphosphates of the specified materials, andpyrophosphoric acid or polyphosphoric acid, may be used. Thus, sodiumpolyphosphate, so called in the literature and commercial use, may beemployed because its chemical structure encompasses the pyrophosphate.Certain of such other materials or substances, however, require additionof acid to provide the desired result. Thus, for example, barium,magnesium, or calcium pyrophosphate may be dispersed in water anddissolved therein by hydrochloric, sulphuric, or other suitable acid.Mixtures of the several substances mentioned can be employed if desired.Orthophosphates and orthophosphoric acid do not produce the desiredresults. The invention therefore contemplates the use of acidifiedsubstances included in the group consisting of the sodium, potassium,lithium, ammonium, and amine pyrophosphates and polyphosphates and metalsubstitution products thereof, pyrophosphoric acid and polyphosphoricacid, and mixtures thereof, and other compounds or agents, which yieldthe pyrophosphate negative radical.

While the invention is applicable to practically all metals, it iscontemplated as being of greatest use in connection with iron, zinc,cadmium, and aluminum and alloys thereof, since corrosion effects ordifiiculty with paint adhesion on other metals are ordinarily of lesspractical importance. The ferrous metals are at present believed toprovide the greatest and most important field for the invention, becauseof their widespread use and susceptibility to corrosion.

In general, the method employed to practice the invention comprises,after cleaning the metal surface which is to be coated, subjecting thecleaned surface to the action of a solution of one or more of theacidified substances described hereinabove, and then rinsing thesurface. If desired, the coated surface may additionally be rinsed witha solution of a substance serving as a sealer for the coating, as forexample chromic acid, where the greatest possible protection againstcorrosion is to be achieved because of expected subjection of thesurface to extreme conditions. In cleaning the surface, it is onlynecessary to remove dirt, film, oil, and the like, while the products ofcorrosion which has already occurred, as for example rust on ferrousmetals, may be allowed to remain because the coating solution willitself etfect the removal thereof. It is to be noted, however, that inthe interests of economy and speed in production, it may be moreadvantageous to remove heavy scale and rust or other corrosion beforeapplication of the coating solution. The cleaning of the surface may beaccomplished in any desired manner and by any appropriate means, and itwill be apparent that such cleaning is a step preliminary to the actualmethod of the invention rather than a step in the method itself, and inmany cases may be dispensed with because the metal surface is alreadysuificiently clean.

Describing the method in greater detail, it may be said that it involvesfour main factors in addition to the particular material employed toobtain the desired coating. These are the quantity of the material usedin the solution, or the concentration, the acidity of the solution, thetemperature of the solution, and the period of time advantageous undergiven conditions.

during which the metal is subjected to the solution. Only relativelysmall quantities of the material are required, so that a lowconcentration of the material in the solution is sufficient, but a highconcentration may be employed if desired or advantageous underparticular conditions. Thus, a saturated solution may be provided. Ifthe surface to be coated is corroded and the corrosion is to bedissipated or removed in the course of providing the coating, a somewhathigher concentration is advisable than if no corrosion is present. Theacidity of the solution may vary through a wide range, but if it is toohigh dissolution of the metal will occur and the resultant pitting willaffect the desired coating adversely, while if it is too low, the timerequired to form the coating will be considerably increased. The desiredacidity is also determined to some extent by the resistance to acidicattack of the particular metal being coated, since if that be notreadily affected by acids, a solution of relatively high pH or lowacidity will require a longer time to effect coating of the metal thanone of higher acidity. The concentration and pH of course vary together,but are not dependent on each other, since the pH may be adjusted by theaddition of alkaline or acid materials to the solution. The reactionbetween the solution and the metal is accelerated by heat, andaccordingly it is ordinarily desirable to raise the solution to a fairlyhigh temperature, which may be as high as the boiling point of thesolution. If time is not an appreciable factor, however, the method maybe carried out with the solution at room temperature or even approachingits eutectic point if desired. The quantity of the coating-productivesubstance which will go into solution in the water or other solventincreases, of course, with increased temperature. The period ofsubjection of the metal to the solution varies with the strength,acidity, and temperature of the solution, as already indicated, and alsowith the weight of coating to be obtained. Ordinarily, a period of a fewminutes is employed. Water or water with a slight admixture of alcohol,has been found eminently satisfactory as a solvent, although theinvention contemplates the use of any solvent employable with any of thematerials described.

Since the various factors affecting the formation of the coating mayvary widely while providing eminently satisfactory resul'ts,'the methodof this invention is specifically applicable to a diversity ofparticular conditions and is readily adaptable within the general limitsset forth hereinto a great variety of conditions and requirements. Theconcentration is best expressed in terms of a pyrophosphate radicalyielded or provided in the substance or material employed, since thedifferent substances set forth hereinabove vary in the yield of theradical for a given quantity of the substance. The selected substance,or mixture of substances, is dissolved in water or other suitablesolvent in a quantity to provide the desired concentration ofpyrophosphate ions, which may range from as low as 0.05% to thesaturation point, as may be most Ordinarily a concentration of the-orderof 1% to is practical for most applications of the invention, and it hasfurther been found in practice that a pyrophosphate ion concentration inthe range between 2% and 4'%, say about 3% produces a very satisfactorycoating while allowing other factors afiecting the coating to varyconsiderably, and this range is therefore preferable for mostoperations. The pH of the solution may-vary-betweenabout l and about 7,and

ordinarily is best held between about 2 and about 5. In

most cases, the pH of the solution requires adjustment by the additionof a suitable acid, for example pyrophosphoric acid, since therelatively -low concentrations usually employed do not provide asolution of the desired acidity. Orthophosp'horic acid is not employed'to adjust the acidity of the solution because it introduces undesirableions. The temperature at which the solution is Cir ing and boilingpoints of the solution, as brought out hereinabove, but warm to hottemperatures, say between F. and 200 F., have been found most useful inpractice, particularly those in the range between about F. or F., and180 F. or 190 F., and this range is therefore preferred under mostconditions. The period of application of the solution to the metal is inpractice usually desired to be only a few minutes, say from about two toabout ten minutes, although it may vary from a matter of seconds to oneof hours, days, or even weeks, depending upon the particularcharacteristics of the solution employed, especially the temperature. Ingeneral, a longer time of subjection of the metal surface to thesolution at a moderate temperature results in a finer, denser coatingthan a shorter time at a higher temperature. Any suitable substancesynergistic to the phosphatization of the metal may be added to theparticular substance employed for producing the coating.

After the metal surface has been coated by the method described, it maybe removed from the solution and any adhering solution removed, as byrinsing with hot water, and then dried. The coating produced protectsthe surface most effectively against corrosion, being integral with themetal, continuous, dense, heavy, and impervious to corrosive agents. Itpromotes bonding of paint, varnish, and like films to the metal,improves the lubricity of the surface for use under frictionalconditions, facilitates drawing, cold extrusion, or like metal formingoperations, and imparts a decorative finish to the metal. Although thecoating is continuous, fine, dense, uniform, relatively heavy, andsubstantially impervious to corrosive agents, it may be sealed so as toaiford additional corrosion protection or the like under extremeconditions by applying a slightly acid solution of a suitable sealer,preferred substances for this purpose being chromic acid, potassiumchromate, and sodium chromate and mixtures thereof.

One specific manner of practicing the method is exemplified by thefollowing procedure, in connection with a ferrous metal surface. Aftercleaning of the surface, it is immersed in or sprayed with an aqueoussolution of sodium acid pyrophosphate in a concentration of about twoounces per gallon of water equivalent to a pyrophosphate ionconcentration of approximately 1.18%, having a pH between 2 and 5, forexample about 3, and heated to a temperature of about F. 'Pyrophosphoricacid may be added to increase the acidity to the desired pH. The surfaceis exposed or subjected to the solution for a period of about twenty orthirty minutes, then rinsed with hot water and dried. This produces afine, dense, uniform, integrally bonded, continuous, coating on thesurface which comprises a mixture of phosphates of iron, resulting fromthe reaction or deposition of the pyrophosp'hate ion with or on themetal, having a weight per square foot ofsurface of some two hundredmilligrams or more. Prior methods of vobtaining iron phosphate coatings.on ferrous metal surfaces have been found incapable of consistentlyproducing coatings in excess of a weightof only about fifty toseventy-five milligrams :per square foot, and the density-and uniformityof such coatings have been greatly inferior to those of the coatings.provided by this invention.

Another specific example of the application of the method to ferrous.metal, in this instance in connection with ferrous metal articleshaving surfaces subject to rusting in a very short time, is illustratedby the following procedure. The articles, sufiicientlyclean-to requireonly a preliminary rinsing :with hot water, are first subjected, tfOI' a:period of about-three minutes, to a solutionof about .four ounces ofsodium :acid pyrophosphate per gallon of water, at a temperatureofabout-1 60 ,F. to F., the pHof the solution being about .3 or 4. Thepyrophosphate ion concentration is about 2.36%. This provides a ""flashor light coating ,on .the metal base. The reason -for the application ofthis -flash.coating is primarily ,to

remove rust formed on the ferrous metal surface in the interval afterprior operations thereon, such as machining, and/ or the cleaning orrinsing, since the solution dissolves or dissipates rust beforedeposition of the coating over rusted areas begins, by reaction with therust to convert it into iron phosphates. The relatively light flashcoating then protects the surface against rusting before the fulldesired coating is produced. After the provision of the flash coating,the surfaces are subjected to another aqueous solution of sodiumpyrophosphate at substantially the same temperature as the first, 160 to170 F., in a concentration of about five ounces per gallon, that is, aconcentration of the pyrophosphate ions of about 2.94%, with a pH ofabout 2 to about 3 controlled by the addition of pyrophosphoric acid,for a period of about five minutes. A coating integral and merged withthe flash coating is produced, giving a complete coating ofapproximately two hundred milligrams per square foot of surface, whichis fine, dense, continuous, tightly-adhering, and uniform. The articlesare then rinsed with hot water, preferably at least at 180 R, which maybe followed by rinsing with a slightly acid hot aqueous solution ofchromic acid, potassium chromate, or sodium chromate or mixturesthereof.

The coatings of the present invention have been sufficiently describedhereinabove as to their characteristics, purposes, effects, andadvantages, as well as the manner in which they are produced, and it isnot believed that further explanation of them is necessary. It is to benoted, however, that it has long been desired to obtain protective andbonding iron phosphate coatings on ferrous metals appreciably heavierthan the fifty to seventyfive milligrams per square foot heretoforeachieved by the prior art, but efforts to provide such heavier coatingshave been unsuccessful. By the present invention, such coatings well inexcess of a hundred milligrams per square foot are readily provided onferrous metals, and in fact coating weights only slightly more than ahundred milligrams per square foot are relatively light compared to thecoating weights obtainable by the invention.

What is claimed is:

1. The method of providing a permanent iron phos phate coating of aweight in excess of one hundred milligrams per square foot on a ferrousmetal surface, which comprises subjecting the metal defining the surfacefor a period of from about one minute to about forty minutes to theaction of a solution consisting essentially of a quantity of an acidpyrophosphate selected from the group consisting of the sodium,potassium, lithium, ammonium, and amine pyrophosphates and mixturesthereof to provide a concentration of pyrophosphate ions from about 0.5%to about 5%, said solution having a pH between about 1 and about 7 and atemperature between about 130 F. and about 200 F.

2. A coated article comprising a ferrous metal base having a coatingcovering a surface thereof and integrally bonded thereto, said coatingbeing of iron phosphates of a Weight in excess of one hundred milligramsper square foot of metal surface and obtained by reaction of the metalwith a solution consisting essentially of an acid pyrophosphate selectedfrom the group consisting of sodium, potassium, lithium, ammonium, andamine pyrophosphates and mixtures thereof obtained by the method ofclaim 1.

3. The method of providing a permanent phosphate coating on'a metal ofthe group consisting of iron, zinc, cadmium, and aluminum and alloysthereof, which comprises subjecting the metal for a period of from aboutone minute to about one hour to a solution consisting essentially of aquantity of an acid pyrophosphate selected from the group consisting ofsodium, potassium, lithium, ammonium, and amine pyrophosphates andmixtures thereof to provide a concentration of pyrophosphate ions of atleast 0.05%, said solution having a pH between about 1 and about 7 andbeing at a temperature in the range between about F. and the boilingpoint of the solution.

4. A coated article comprising a base of metal of the group consistingof iron, zinc, cadmium, and aluminum and alloys thereof which has acoating covering a surface thereof and integrally bonded thereto, saidcoating being of a complex mixture of phosphates of the metal obtainedby reaction of the metal with a solution consisting essen tially ofsodium acid pyrophosphate obtained by the method of claim 3.

References Cited in the file of this patent UNITED STATES PATENTS2,067,007 Darsey Jan. 5, 1937 2,337,856 Rice et al. Dec. 28, 19432,516,685 Douty et al. July 25, 1950 2,552,874 Snyder et al. May 15,1951 2,609,308 Gibson Sept. 2, 1952 2,692,189 Ro Oct. 19, 1954 FOREIGNPATENTS 127,347 Australia Apr. 7, 1948

1. THE METHOD OF PROVIDING A PERMANENT IRON PHOSPHATE COATING OF AWEIGHT IN EXCESS OF ONE HUNDRED MILLIGRAMS PER SQUARE FOOT ON A FERROUSMETAL SURFACE, WHICH COMPRISES SUBJECTING THE METAL DEFINING THE SURFACEFOR A PERIOD OF FROM ABOUT ONE MINUTE TO ABOUT FORTY MINUTES TO THEACTION OF A SOLUTION CONSISTING ESSENTIALLY OF A QUANTITY OF AN ACIDPYROPHOSPHATE SELECTED FROM THE GROUP CONSISTING OF THE SODIUM,POTASSIUM, LITHIUM, AMMONIUM, AND AMINE PYROPHOSPHATES AND MIXTURESTHEREOF TO PROVIDE A CONCENTRATION OF PYROPOSPHATE IONS FROM ABOUT 0.5%TO ABOUT 5%, SAID SOLUTION HAVING A PH BETWEEN ABOUT 1 AND ABOUT 7 AND ATEMPERATURE BETWEEN ABOUT 130*F. AND ABOUT 200*F.